Even so, in an advisory issued late Wednesday and obtained by ABC News, FBI and Homeland Security officials warned of "renewed efforts to target Western aviation."

European law enforcement officials said stepped up security was being planned at major airports and transportation hubs over the next several days.

"While there is no credible threat, there is much preparation based on the common sense consideration of the date," said one intelligence official.
Officials told ABC News there are several uncorroborated threats against U.S. interests, including some on the internet, that are being investigated but so far have low credibility.

"We assess that such threats are almost certainly aspirational and are not indicative of actual plotting," the law enforcement advisory said.

The killing of bin Laden by U.S. Navy SEALs led to numerous calls for attacks on the United States to avenge the terror leader's death.

The law enforcement advisory acknowledged al Qaeda would regard an attack on the U.S. "as a symbolic victory that would help reassert the group's global relevance following the major leadership losses and operational setbacks it has suffered over the past year."

American law enforcement officials tell ABC News they regard the al Qaeda affiliates in Yemen and Somalia as the most likely to be able to carry out an attack on the United States.

The Yemen-based group known as al Qaeda of the Arabian Peninsula (AQAP) has attempted two attacks against U.S.-bound aircraft, according to the FBI, and "represents an enduring threat to the West."